Applicant claims priorities under 35 U.S.C. xc2xa7119 of German Application Nos. 199 42 559.0, filed: Sep. 7, 1999, 199 51 137.3, filed: Oct. 23, 1999, and 100 00 280.3, filed: Jan. 7, 2000. Applicant also claims priority under 35 U.S.C. xc2xa7120 of PCT/EP00/06488, filed: Jul. 8, 2000. The international application under PCT article 21(2) was not published in English.
The invention relates to a process for the methanol synthesis from hydrogen, carbon monoxide and carbon dioxide under pressure, in particular for increasing the yield of processes already in use, whereby desulphurized natural gas is charged in a reformer and the synthesis gas is subsequently admitted to a methanol synthesis.
A number of devices or processes for the catalytic methanol synthesis are known, whereby the following documents are cited here as examples of the abundance of solutions: DE 21 17 060; De 25 29 591; DE 32 20 995; DE 35 18 362; U.S. Pat. No. 2,904,575; and DE 41 00 632.
A methanol production plant is normally operated in conjunction with a plant for producing synthesis gas, whereby both plants are dimensioned in such a way that the synthesis gas produced exactly covers the requirements of the methanol-producing catalyst of the synthesis gas circulation and in particular is stoichiometrically composed based on the following reactions, whereby only two of said reactions are linearly dependent upon each other:                     CO        +                  2          ⁢                      xe2x80x83                    ⁢                                    H              2                        ⟷                          CH              3                                ⁢          OH                ⁢                  xe2x80x83                -                  90.84          ⁢                      xe2x80x83                    ⁢                      kJ            /            mol                                              (        1        )                                                      CO            2                    +                                    H              2                        ⟷            CO                    +                                    H              2                        ⁢            O                    +                      41.20            ⁢                          xe2x80x83                        ⁢                          kJ              /              mol                                      ⁢                  xe2x80x83                                    (        2        )                                                      CO            2                    +                      3            ⁢                          xe2x80x83                        ⁢                                          H                2                            ⟷                              CH                3                                      ⁢            OH                    +                                    H              2                        ⁢            O                    -                      49.64            ⁢                          xe2x80x83                        ⁢                          kJ              /              mol                                      ⁢                  xe2x80x83                                    (        3        )            
According to the above reaction equations, the following applies to a stoichiometric synthesis gas:                     ζ        =                                                            H                2                                                                    C                            -                              CO                2                                                                    C                                                                                      xe2x80x83                                C                            ⁢              CO              ⁢                              xe2x80x83                            ⁢                              +                C                            ⁢                              CO                2                                              =          2                                    (        4        )            
whereby ci is the gas concentration of the respective starting materials based on moles.
The subscript xe2x80x9cixe2x80x9d represents the individual compounds. Alternatively, ci can be written as set forth above, thus xe2x80x9ccixe2x80x9d can refer to: cH2, or cCO2, or cCO, or cCO2.
Such a synthesis gas is usually produced in one single production line in a primary reformer, or together with a secondary reformer or in similar gas production equipment.
It is difficult to refit such a plant at a later time if existing production capacities have to be expanded. Generally, another production plant has to be built in such a case based on the layout of the old one, and no synergy effects can be expected. Furthermore, tying-in synthesis gas from external sources, which is often available at low cost when other larger plants are shut down, poses problems in that because of different compositions of the gas, which in most cases is an excessive component of carbon-containing compounds (xe2x80x9cC-componentxe2x80x9d), such gas cannot be used for the production of methanol without further processing requiring substantial expenditure.
Therefore, the problem of the invention is to expand the known process by overcoming the drawbacks that have become known so far, creating good possibilities for later refitting of existing plants, and to make it possible to employ synthesis gas from other sources.
With a process of the type specified above, said problem is solved according to the invention in that after passing through the reformer, a side stream from the stream of synthesis gas is supplied to a methanol pre-reactor; the methanol produced in the pre-reactor is fed into the methanol stream exiting from the methanol synthesis of the main stream; and a stream of synthesis gas that has not been reacted in the methanol pre-reactor is recycled into the main stream prior to the methanol synthesis, whereby additional synthesis gas compensating the incurred loss is simultaneously charged within the zone where the non-reacted synthesis gas is recycled.
The methanol pre-reactor employed in the process, as well as the devices usually employed for conditioning the synthesis gas, as well as for the condensation and separation of the methanol produced can be refitted at a later time, whereby it is possible in this way to increase the production capacity up to 57% based on the capacity of the old installation. Of course, this concept is not limited to the later refitting of old plants, but it can be advantageously taken into account in the new conception as well.
Further developments of the invention follow from the dependent claims. Provision can be made in this conjunction for using as the additional synthesis gas a synthesis gas from external sources, i.e. from a separate synthesis gas source, or a synthesis gas that is withdrawn from the stream of natural gas as a bypass and then passed via an xe2x80x9cautothermxe2x80x9d reformer, or a synthesis gas that originates from another synthesis gas production.
The foreign synthesis gas may originate in this connection from a combined xe2x80x9cautothermxe2x80x9d reformer (also referred to as xe2x80x9cCARxe2x80x9d), or from a reactor for partial oxidation, which can be refitted at a later time and is substantially more favorable in terms of construction than the apparatuses usually employed for producing synthesis gas with an exactly xe2x80x9cfittingxe2x80x9d gas composition. In this case, it becomes possible also jointly use a portion of the exhaust gas of the methanol production plant, which otherwise would be usable only as firing gas, as an additional gas charged in an xe2x80x9cautothermxe2x80x9d reformer, or in a combined xe2x80x9cautothermxe2x80x9d reformer.
It may be advantageous in this conjunction if the additional synthesis gas compensating the loss and fed into the main stream of the synthesis gas prior to the methanol synthesis, is withdrawn from a combined xe2x80x9cautothermxe2x80x9d reformer and/or from a reactor for partial oxidation, or from another synthesis gas production, for which provision is made according to the invention as well.
According to another further development of the invention, the off-heat that has to be dissipated from the methanol pre-reactor during the cooling of the methanol synthesis gas mixture, is exploited for operating an absorption refrigeration machine. The produced cold is used for cooling the synthesis gas exiting from the reformers before it is compressed, so that compression energy is saved in this way.
The saved compression energy can be exploited according to the invention for compressing the synthesis gas obtained from external sources, or the additional synthesis gas produced in refitted reformers. Furthermore, the produced cold can be used for condensing out more methanol downstream of the methanol reactors. If the objective is to primarily save investment costs, and if the energy costs are low, it may be economical also to dispense with any exploitation of the off-heat and to employ instead of an absorption refrigeration machine a conventional refrigeration machine operated with, for example ammonia as the refrigerant.
With the mixture of the foreign synthesis gas and the synthesis gas obtained from the methanol pre-reactor after the methanol produced has been separated, it is necessary to ensure that the mixture approximately corresponding with the composition of the synthesis gas that has been originally branched off, fully covers the conditions for the existing methanol circulation, and that the original synthesis can be operated within the framework of the catalyst conditions. In order to assure this, at least the following must apply to the molar concentrations of the foreign synthesis gas: 0.8xe2x89xa6xcex6xe2x89xa64. An improvement is obtained if the following applies additionally: 1xe2x89xa6xcex6xe2x89xa62.5. The greater the proportion of the withdrawn side stream and the methanol produced in the pre-reactor, the more careful hone has to make sure that the characteristic xcex6 of the foreign synthesis gas approaches number 2.
A further development of the invention consists in that a mixture of H2 and CO2 is used as the additional synthesis gas, whereby the CO2 present in the mixture originates from the flue gas of a firing installation or from the exhaust gas of a CO2 washing operation in an ammonia plant (FIG. 2).
Using in the additional synthesis gas the CO2 originating from a combustion process offers the great advantage that said CO2 does not have to be discharged into the atmosphere, but is employed as intended for the synthesis.
Provision is made in this connection for the further development that the CO2 from a flue gas purification plant originates from a firing operation, for example from the firing of the primary reformer.
Said procedure offers the additional benefit that the climatically active CO2 is introduced into the substance circulation, so that a fee charged, for example for xe2x80x9chothouse emissionsxe2x80x9d needs no longer to be paid, which renders the procedure substantially more economical.
In addition to using the CO2from a combustion plant, such CO2 may originate from the CO2 washing operation of an NH3 plant as well. Such an ammonia plant is not explained in greater detail in the following.
For solving the problem specified above, the invention makes provision also for a device for the methanol synthesis, in particular a device for carrying out the process according to any one of the preceding claims, such a device comprising a natural gas supply (or feed) line leading to a reformer, and a methanol synthesis connected downstream, whereby such a device as defined by the invention is characterized by the following:
a branched-off line for a side stream of the synthesis gas exiting from the secondary reformer;
a methanol pre-reactor in the side stream;
a methanol feed line leading to the methanol main stream exiting from the methanol synthesis;
a recycling line recycling non-reacted synthesis gas from the methanol pre-reactor into the synthesis gas main stream upstream of the methanol synthesis; as well as
a feed line upstream of the methanol synthesis for feeding synthesis gas compensating losses.
As a further development of the invention, the device as defined by the invention is characterized in that an xe2x80x9cautothermxe2x80x9d reformer is connected in parallel with the reformer, whereby the exiting synthesis gas is at least partly used as synthesis gas compensating losses.
According to the invention, provision can be made also for a flue gas washing operation for the flue gas exiting from the primary reformer, as well as for a CO2feed line for supplying the foreign or additional gas, as well as for a feed line for feeding foreign H2 (FIG. 2), whereby it may be useful if provision is made for a supply line for feeding foreign CO2 from a CO2 washing operation in the NH3 plant, as well as for a supply line for feeding foreign H2, for which lines provision is made in further development of the invention as well.